1. Field of the Invention
The present invention relates to an optical amplifier and method for amplifying optical signal light using an optical fiber doped with a rare earth element, and especially for amplifying optical signal light in a long wavelength band.
2. Description of the Related Art
With the advancing development of multimedia networks, demand for information is drastically increasing. Therefore, trunk optical transmission systems, which have relatively high information transmission capacity, will be required to have even higher information transmission capacity and will be required to form flexible networks.
To provide higher transmission capacity, wavelength division multiplexing (WDM) optical transmission systems are being used. The commercialization of WDM optical transmission systems has already been advanced mainly in North America.
Moreover, WDM optical amplifiers have been used to amplify WDM optical signals. A WDM optical amplifier can collectively amplify signal lights having two or more different wavelengths in, for example, a wavelength range of 1.53 to 1.57 .mu.m (hereinafter referred to as the "conventional band"). Therefore, the use of WDM optical amplifiers in a WDM optical transmission system can enable high-capacity, long-distance optical transmission with a relatively simple configuration.
Furthermore, by expanding the wavelength band of an optical amplifier, a system has been proposed which employs a long wavelength band, for example, of 1.57 to 1.62 .mu.m (hereinafter referred to as the "longer wavelength band"), as a new transmission band.
The following is a description of the use of an optical amplifier employing an erbium doped fiber (EDF) for the amplification of signal light in the longer wavelength band.
FIG. 1 is a graph showing the gain per unit length versus wavelength characteristics of an EDF corresponding to the degrees of population inversion (ranging from 0.0 to 1.0). As shown in FIG. 1, in the conventional band, the gain characteristic of an EDF is flat in the case that the degree of population inversion is 70% or so. Namely, the gain of an EDF in the conventional band is dominant.
Conversely, in the longer wavelength band, the gain characteristic of an EDF is flat in the case that the degree of population inversion is low, namely, 40% or so. Thus, the gain of an EDF in the longer wavelength band is dominant. Therefore, in the case of optical amplification of signal light in the longer wavelength band, an excitation light of a wavelength in a 0.98 .mu.m band or a 1.48 .mu.m band is supplied to the EDF by setting the degree of population inversion at a low level. In this case, the amplification factor per unit length of the EDF decreases in principle because of the low degree of population inversion.
In the forward excitation case of supplying the excitation light from a signal light input terminal of the EDIF to a signal output terminal thereof, the degree of population inversion corresponds to the excitation light power. Therefore, the degree of population inversion is high at the signal light input terminal, but is low at the signal light output terminal. In the backward excitation case of supplying an excitation light from the signal light output terminal of the EDF to the signal input terminal thereof, there is a relation between the degrees of population inversion at the signal input and output terminals thereof opposite to that of the forward excitation case.
Thus, generally, a required total gain of the EDF in the case of the optical amplification of signal light in the longer wavelength band is obtained by elongating a conventional EDF used for the optical amplification of signal light of the conventional band, to thereby lower the degree of population inversion. This can be understood by referring to FIG. 2, which illustrates gain distribution in the longitudinal direction of an EDF.
Further, when the degree of population inversion is set at a low level, the absorption of excitation light is increased. For example, if the wavelength of the excitation light is in the 1.48 .mu.m band in FIG. 1, the gain per unit length of the EDF is about -0.2 dB when the degree of population inversion is 0.4. This indicates that, in such a case, the excitation light is more likely to be absorbed in the EDF, as compared with the case where the gain per unit length thereof is about 0 dB when the degree of population inversion is 0.7. When the excitation light is largely absorbed therein, the absorption of excitation light is performed at a biased position in the EDF. Consequently, the excitation power propagation efficiency in the longitudinal direction of the EDF is reduced. Thus, the optical amplification of signal light in the longer wavelength band has a feature that the excitation efficiency of the entire EDF is limited to a low value in comparison with that of the entire EDF in the case of the optical amplification of signal light in the conventional band. A conventional optical amplifier for amplifying signal light of a long wavelength band having such a feature is described in, for example, the article titled "Gain Flattened Er.sup.3+ Doped Fiber Amplifier for a WDM Signal in the 1.57-1.60 .mu.m Wavelength Region," Ono et al., IEEE Photon. Tech. Lett., Vol. 9, pp. 596-598, May, 1997.
FIG. 3 is a diagram showing such a conventional long wavelength band optical amplifier. In the optical amplifier of FIG. 3, an incident longer wavelength band signal light L.sub.s passes through an optical isolator 2.sub.1 and is multiplexed with an excitation light Lp.sub.1, emitted from an excitation light source 4.sub.1 by a wavelength division multiplexing (WDM) coupler 3.sub.1. Then, the multiplexed light enters an EDF 1. An excitation light Lp.sub.2 is emitted from an excitation light source 4.sub.2. At an emitting terminal of EDF 1, excitation light Lp.sub.2 is multiplexed by a WDM coupler 3.sub.2 and is then propagated through EDF 1 in the opposite direction than excitation light Lp.sub.1, thereby contributing to optical amplification. The longer wavelength band signal light L.sub.s passes through WDM coupler 3.sub.2 and optical isolator 2.sub.2 after passing through EDF 1. Then, signal light L.sub.s is emitted from the amplifier.
FIG. 4 is a diagram showing another conventional long wavelength band optical amplifier. Such a long wavelength band optical amplifier is described, for example, in the article titled "Amplification Characteristics of 1.58 .mu.m Band Er.sup.3+ Doped Fiber Amplifier," Ono et al., TECHNICAL REPORT OF IEICE, Vol. OCS97-5, pp. 25-30, 1997.
In the optical amplifier of FIG. 4, an incident longer wavelength band signal light L.sub.s passes through an optical isolator 2.sub.1. A forward excitation light Lp.sub.1 is emitted from an excitation light source 4.sub.1. Signal light L.sub.s and excitation light Lp.sub.1 are multiplexed by a WDM coupler 3.sub.1. Then, the multiplexed light enters a pre-stage EDF 1.sub.1. After passing through pre-stage EDF 1.sub.1, the longer wavelength band signal light L.sub.s passes through an optical isolator 2.sub.3 and enters a post-stage EDF 1.sub.2. Further, a backward excitation light Lp.sub.2 is emitted from an excitation light source 4.sub.2 Excitation light Lp.sub.2 enter post-stage EDF 1.sub.2 through a WDM coupler 3.sub.2 Excitation light Lp.sub.2 propagates through post-stage EDF 1.sub.2 in the opposite direction than excitation light Lp.sub.1, and thereby contributes to the optical amplification by the post-stage portion. Then, the longer wavelength band signal light L.sub.s passes through WDM coupler 3.sub.2 and an optical isolator 2.sub.2 after passing through post-stage EDF 1.sub.2. Finally, signal light L.sub.s reaches the emitting terminal of the amplifier. In this case, one of wavelengths (ranging from 960 to 1000 nm) in the 0.98 .mu.m band is used as that of the forward excitation light Lp.sub.1. Moreover, one of wavelengths (ranging from 1450 to 1490 nm) in the 1.48 .mu.m band is used as that of the backward excitation light Lp.sub.2. With this configuration, a low noise optical amplifier is realized.
FIG. 5 is a diagram showing another conventional long wavelength band optical amplifier. Conventional long wavelength band optical amplifiers, such as that in FIG. 5, are described, for example, in the article titled "Low Noise Operation of Er.sup.3+ Doped Silica Fiber Amplifier around 1.6 .mu.m," Massicott et al., Electron. Lett., Vol. 28, pp. 1924-1925, September 1992, and U.S. Pat. No. 5,500,764 Official Gazette.
In the optical amplifier of FIG. 5, an incident longer wavelength band signal light L.sub.s is multiplexed with conventional band signal light Lp.sub.3 through a multiplexer 5, passes through an isolator 2.sub.1, and is then multiplexed at a WDM coupler 3.sub.1 with an excitation light Lp.sub.1 emitted from an excitation light source 4.sub.1. Then, the multiplexed signal light L.sub.s enters in EDF 1. At an emitting terminal of EDF 1, an excitation light Lp.sub.2 emitted from an excitation light source 4.sub.2 is multiplexed at a WDM coupler 3.sub.2 Excitation light Lp.sub.2 propagates through EDF 1 in the opposite direction than excitation light Lp.sub.1, and thereby contributes to amplification. The longer wavelength band signal light L.sub.s passes through WDM coupler 3.sub.2 and an optical isolator 2.sub.2 after passing through EDF 1. Finally, signal light L.sub.s reaches the emitting terminal of the amplifier. This optical amplifier improves the excitation efficiency by adding the conventional band signal light Lp.sub.3 to the signal light L.sub.s at a low level. Namely, the more the excitation light having a wavelength close to that of signal light is used, the higher the excitation efficiency. Therefore, the conventional band light is used as excitation light for amplifying the longer wavelength band signal light.
However, the conversion efficiency of the conventional amplifier in FIG. 3 is only 37.7% or so when the gain flattens in the case that the wavelength of the excitation light is set at, for example, one of wavelengths of 1450 to 1490 nm. Therefore, this conventional optical amplifier has a problem that properties, such as excitation efficiency and noise factor, are inferior to those in the case of optical amplification of signal light of the conventional band.
To cope with this problem, the conventional optical amplifier in FIG. 4 reduces the length of pre-stage EDF 1.sub.1, increases the length of post-stage EDF 1.sub.2 and uses the 0.98 .mu.m band excitation light, which has good noise characteristics, for the pre-stage portion amplification. Thus, the noise level in the case of this conventional optical amplifier is low, as compared with the optical amplifier of FIG. 3.
However, although the optical amplifier in FIG. 4 is effective in reducing noise, this optical amplifier has problems in that the excitation power transmission efficiency is low and that the excitation efficiency is still low.
In contrast, the optical amplifier in FIG. 5 obtains high excitation efficiency by supplying the conventional band light Lp.sub.3 to EDF 1, and reduces the energy consumption thereof. However, this conventional optical amplifier has problems in that an additional light source for generating the conventional band light is required and that active optical parts, such as a light source, are expensive and thus the cost of the amplifier is increased.